(1) Field of the Invention
The present invention relates to a method for desulfurizing an exhaust gas in a wet lime/gypsum process exhaust gas desulfurizing apparatus, and to a method for treating an exhaust gas containing SO.sub.2 and HCl together.
(2) Description of the Prior Art
A wet lime/gypsum process exhaust gas desulfurizing apparatus for removing SO.sub.2 in an exhaust gas therefrom by the use of CaCO.sub.3 as an absorbent is most prevalent for the exhaust gas desulfurization of late, and this type of apparatus is described in detail in many publications. For the purpose of directly recovering gypsum as a by-product from an SO.sub.2 gas absorption tower, a method for blowing air into an absorbing solution is disclosed in Japanese Patent Publication No. 17,318/1975. Here, reference will be made to the conventional method in accordance with FIG. 2.
In FIG. 2, an exhaust gas inlet 2 is provided at an upper portion of a tower body 1, and an exhaust gas outlet 3 is provided at an lower portion thereof. In the tower 1 and under the exhaust gas outlet 3, there is an absorbing solution storage section 5, in which a stirrer 4 and an air introduction pipe 8 are disposed. A spray nozzle 7 for spraying a slurry containing Ca compounds is disposed at an upper portion of the tower 1, and is connected to the absorbing solution storage section 5 via a pipe provided with a circulating pump 6 in the middle thereof.
The exhaust gas containing SO.sub.2 is introduced into the tower 1 through the exhaust gas inlet 2, flows downward in the tower 1, and is discharged through the exhaust gas outlet 3. On the other hand, the slurry jetted through the spray nozzle 7 flows downward through a grid filler 9, while brought into contact with the exhaust gas. Then, in the absorbing storage section 5, the slurry is brought into contact with air fed through the air introduction pipe 8, while stirred by the stirrer 4, and it is delivered to the spray nozzle 7 by means of the circulating pump 6 again.
On the other hand, CaCO.sub.3 which is a kind of absorbent is fed through an absorbent feed pipe 11 in compliance with an amount of absorbed SO.sub.2, and thus the absorbing solution which has become acidic by the absorption of SO.sub.2 is thereby neutralized to calcium sulfite, which is further oxidized to calcium sulfate (gypsum). The thus formed calcium sulfate must be discharged from the system on the basis of material balance. Therefore, a part of the absorbing solution is drawn out from the absorbing solution storage section 5 through an absorbing solution draw pipe 12.
The slurry which has been sprayed through the spray nozzle 7 and which will be brought into contact with the exhaust gas contains calcium sulfate as the main component, and unreacted CaCO.sub.3 and some unoxidized calcium sulfite. By absorbing SO.sub.2, an acidic sulfite is produced in the slurry and the latter falls onto the surface 10 of the solution in the absorbing solution storage section 5.
The part of the absorbing solution drawn out through the absorbing solution draw pipe 12 is delivered to a filter device in order to recover the secondarily produced gypsum, but for the purpose of heightening the purity of the by-product gypsum, it is necessary to lower the concentrations of unreacted CaCO.sub.3 and unoxidized calcium sulfite.
However, in the conventional process, the one tank type absorbing solution storage section 5 is used, and while CaCO.sub.3 which is the alkaline absorbent is fed through the absorbing solution feed pipe 11, the air oxidation is simultaneously carried out. Therefore, such a process has the drawback that the unreacted CaCO.sub.3 and the unoxidized calcium sulfite will remain therein inevitably.
Thus, if it is attempted to decrease a feed of the absorbent CaCO.sub.3 and to thereby diminish an amount of the remaining CaCO.sub.3, a pH of the absorbing solution will lower and an SO.sub.2 absorbing performance will deteriorate inconveniently. Further, in order to reduce this inconvenience as much as possible, it has been tried to increase the volume of the absorbing solution storage section, but such a strategy cannot provide any sufficient effect in spite of a great economical loss.